In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, a developing roller and a quantity regulating blade (charging blade) kept in press contact with an outer peripheral surface of a roller body of the developing roller are used for developing an electrostatic latent image formed by exposing an electrically charged surface of a photoreceptor drum into a toner image.
That is, when the developing roller is rotated with the quantity regulating blade in press contact with the developing roller, toner is electrically charged. The electrically charged toner is caused to adhere onto the outer peripheral surface of the developing roller, while the amount of the adhering toner is regulated by the quantity regulating blade. Thus, a toner layer is formed on the outer peripheral surface of the developing roller as having a generally constant thickness.
When the developing roller is further rotated in this state to transport the toner layer to the vicinity of the surface of the photoreceptor drum, the toner of the toner layer is selectively transferred from the toner layer onto the drum surface according to the electrostatic latent image formed on the surface of the photoreceptor drum. Thus, the electrostatic latent image is developed into the toner image.
The developing roller is generally produced in the following manner. A rubber composition is formed into a tubular shape and crosslinked to produce a roller body. Then, a shaft made of a metal or the like is inserted through a center through-hole of the roller body to be electrically connected and mechanically fixed to the roller body. Further, an outer peripheral surface of the roller body is polished as required.
For preparation of the rubber composition, a base rubber containing, for example, at least a copolymer rubber (ion conductive rubber) including ethylene oxide as a comonomer and having ion conductivity is prepared, and additives such as a crosslinking agent and an accelerating agent for crosslinking the base rubber are blended with the base rubber.
The developing roller is required to prevent imaging failures such as a so-called fogging phenomenon in which the toner adheres to a background portion of the formed image, and inconsistent image density of the formed image.
To satisfy the requirement, it is contemplated, for example, in Patent Literature 1 to control an average inter-peak distance Sm at 10 to 300 μm to define the surface roughness of the outer peripheral surface of the roller body, and to control the microscopic rubber hardness A of the roller body at 20 to 80.
Further, it is contemplated in Patent Literature 2 to control a center line average roughness Ra at not greater than 0.3 μm to define the surface roughness of the outer peripheral surface of the roller body, and to control the positional relationship between the quantity regulating blade and the developing roller in a predetermined range.
In Patent Literature 3, it is contemplated to control the center line average roughness Ra, the ten-point average roughness Rz and the roughness curve element average length Rsm of the outer peripheral surface of the roller body with respect to the volume average particle diameter of the toner to satisfy a predetermined relationship.
In Patent Literature 4, it is contemplated to coat the outer peripheral surface of the roller body with a coating layer containing particles of at least an acrylic resin and a fluororesin.